The present invention pertains to the field of integrated circuit design. More particularly, the present invention relates to a power saving termination technique for voltage and current steered differential busses.
A computer system typically has components such as a processor, a main memory, a cache, and a chipset. Components of a computer system communicate with one another through interconnections or busses. There are multiple ways to implement a bus. The type of data to be transferred and timing requirements between computer components are common factors used to decide which bus implementation to use.
The use of differential busses has become more prevalent as the need for extremely high transfer rates between components in a computer system continue to grow. Differential busses typically involve the transfer of a pair of signals, known as a differential pair, such that when data on one transmission line is asserted high, the other transmission line has an active low signal. A receiver receives the signals and looks only at the difference between the two signals. Differential busses help to cancel out noise that is picked up on transmission lines because adjacent wires usually pick up approximately equal noise voltages. The more noise a bus is subjected to, the less timing margin the data is given to propagate across a transmission line. As a result, decreasing the noise on a bus helps a system to achieve improved transfer rates between components.
Several of the latest differential busses such as Infiniband, Third Generation Input/Output (3GIO), Serial Advanced Technology Attachment (SATA), and Universal Serial Bus (USB) use encoding techniques to eliminate direct current (DC) and low frequency components of a signal. Other busses achieve similar results using a modulation technique. By generating an approximately equal number of digital high and digital low signals to be transmitted across a bus, encoding and modulation helps to reduce signal distortion on the bus.
Moreover, encoded and modulated signals save power. For example, if an active high signal has to be driven for a great distance over a long period of time, the transmission line has to be charged for the entire time and distance. In the same example, by forcing intermittent low signals over the transmission line, encoded and modulated signals do not require the transmission line to be continuously charged.
Differential systems, however, are still susceptible to static state conditions such as when the system is placed in a standby mode. During static state, current flows if the voltages on the differential pair are different. As a result, power is dissipated. Thus, in order to conserve power in differential systems, it would be desirable to design a bus circuit that provides a bypass for static current flow while allowing transmitted encoded signals to reach their receiver circuitry.